The present invention relates to a planar type semiconductor device, and more particularly to a planar type semiconductor device with a high breakdown voltage having a field plate and a guard ring.
In general, a breakdown voltage of a planar type semiconductor device is lower than that of a mesa type semiconductor device. The reason for this is that, in the planar type semiconductor device, an electric field is concentrated mainly at a PN junction in the surface area of a semiconductor substrate, so that a breakdown voltage at the PN junction in the surface area is lower than that at the PN junction in the interior of the substrate. It is well known that a field plate or a guard ring is used as the means for improving a breakdown voltage of the planar type semiconductor device. A structure of a PN diode with a field plate, for example, is illustrated in FIG. 1 and a structure of a PN diode with a guard ring is illustrated in FIG. 2. In FIGS. 1 and 2, reference numeral 1 designates an N type semiconductor substrate, 2 a P.sup.+ type diffusion area formed on the surface of the substrate 1, 3 an insulating film of SiO.sub.2, for example, 4 an anode electrode, 5 a cathode electrode. In FIG. 1, a field plate 6 integral with an anode electrode 4 is provided on an insulating film formed outside the P.sup.+ type diffusion area 2. When an inverse bias voltage -V is applied to the anode electrode 4 of the device shown in FIG. 1, a depletion layer formed in the substrate 1 takes a shape as indicated by the broken line. The field plate 6 weakens the intensity of the electric field concentrated at the PN junction in the surface area of the substrate 1, resulting in providing the diode with a high breakdown voltage. Referring to FIG. 2, a P.sup.+ type guard ring region 7 is provided surrounding the P.sup.+ type diffusion area 2 at a given interval therebetween. With this structure, when the depletion layer (as indicated by a dotted line) produced when an inverse bias voltage -V is applied to the anode electrode 4 grows to reach the guard ring region 7, the guard ring region 7 shares a given amount of the voltage, so that an electric field between the diffusion area 2 and the guard ring 7 is kept at a fixed value. An electric field larger than the fixed value is used for further extending the depletion layer beyond the guard ring region 7. Therefore, the concentration of the electric field at the PN junction in the surface area of the substrate 1 is relieved, allowing provision of diodes with a high breakdown voltage. If necessary, a plurality of guard ring regions 7, not a single region, may be provided.
When negative electric charges are attached onto the insulating film, a channel is formed in a part of the surface of the substrate 1 corresponding to the part of the insulating layer with the electric charges attached. The formation of the channel deteriorates the function of the guard ring 7, that is, the function to restrict an electric field between the diffusion region 2 and the guard ring region to a given value. Since the amount of the charge attached to the insulating layer 3 is not always constant, the conductivity of the channel formed by the charge is also not fixed. This results in an instability in the expansion of the depletion layer. In order to remove the instability, Japanese Patent Publication No. 36513/49 (1974) discloses a semiconductor device with a structure as shown in FIG. 3. In FIG. 3, first and second guard ring regions 7a and 7b are provided surrounding the P.sup.+ type diffusion region 2. A field plate 8a kept at a potential equal to that of the guard ring region 7a and a field plate 8b kept at a potential equal to that of the guard ring region 7b are provided on the insulating layer 3, extending toward the diffusion layer 2. In the structure of the semiconductor device, when an inverse bias voltage is applied to the anode electrode 4, the guard ring regions 7a and 7b share given voltages. Therefore, the electric field concentration at the PN junction in the surface area of the diffusion region 2 is prevented. Further, the field plates 8a and 8b are kept at the potential equal to that of the guard ring regions 7a and 7b. For this reason, the instability of the expansion of the depletion layer due to the charge attachment to the insulating film 3 is prevented, providing stable semiconductor devices with a high breakdown voltage.
The semiconductor device shown in FIG. 3, however, has the following drawbacks. Firstly, there are optimum values in the distances between the diffusion region 2 and the first guard ring region 7a and between the first guard ring region 7a and the second guard ring region 7b. If the distances do not fall within the optimum values, respectively, a high breakdown voltage characteristic can not be obtained. Further, it is difficult to secure the optimum values of the distances in manufacturing the semiconductor devices. Secondly, when a low ON-resistance of the semiconductor device is required, the resistance of the semiconductor substrate 1 must be set small. It is as a matter of course that when the resistance of the substrate is small, the expansion of the depletion layer is small. Meantime, the field plates 8a and 8b are extended in a direction to interrupt the expansion of the depletion layer. In order that the guard ring regions 7a and 7b share given voltages, the distance between the diffusion layer 2 and the guard ring region 7a and the distance between two guard ring regions must be shortened. This will be discussed in detail. Let us consider a case to manufacture a semiconductor device in which the resistance of the substrate 1 is below 18 .OMEGA..multidot. cm and the breakdown voltage is the order of several hundred volts. A thickness of the depletion layer extending from the diffusion layer 2 to the inner side of the substrate 1 when an inverse bias voltage is applied to the anode electrode 4, is small, compared to the case where the substrate 1 has the high resistance. Therefore, a desired high breakdown voltage can not be obtained of course. In order to make the depletion layer expand from the diffusion layer 2 to the guard ring region 7a before the inverse bias voltage reaches the breakdown voltage at the PN junction, the distance between the diffusion area 2 and the guard ring region 7a must be set small. For the same reason, the distance between the guard ring regions 7a and 7b must be set small. Therefore, the manufacturing of the semiconductor devices is difficult and the production yield of the semiconductor devices is decreased.